This invention relates to methods of processing a plurality of image charge/current signals representative of trapped ions undergoing oscillatory motion, e.g. for use in an ion trap mass spectrometer. The invention also relates to associated methods and apparatuses.
In general, an ion trap mass spectrometer works by trapping ions such that the trapped ions undergo oscillatory motion, e.g. backwards and forwards along a linear path or in looped orbits.
An ion trap mass spectrometer may produce a magnetic field, an electrodynamic field or an electrostatic field, or combination of such fields to trap ions. If ions are trapped using an electrostatic field, the ion trap mass spectrometer is commonly referred to as an “electrostatic” ion trap mass spectrometer.
In general, the frequency of oscillation of trapped ions in an ion trap mass spectrometer is dependent on mass/charge ratio of the ions, since ions with large mass/charge ratios generally take longer to perform an oscillation compared with ions with small mass/charge ratios. Using an image charge/current detector, it is possible to obtain, non-destructively, an image charge/current signal representative of trapped ions undergoing oscillatory motion in the time domain. This image charge/current signal can be converted to the frequency domain e.g. using a Fourier transform (“FT”). Since the frequency of oscillation of trapped ions is dependent on mass/charge ratio, an image charge/current signal in the frequency domain can be viewed as mass spectrum data providing information regarding the mass/charge ratio distribution of the ions that have been trapped.
Fourier transform ion cyclotron resonance (“FTICR”) is a known mass spectrometry technique which employs a superconductor magnetic field for ion trapping and implements these principles.
A known example of an electrostatic ion trap mass spectrometer is the “Orbitrap”, developed by Alexander Makarov. In an Orbitrap, ions trapped by an electrostatic field cycle around a central electrode in spiral trajectories.
Another known example of an ion trap mass spectrometer is the electrostatic ion beam trap (“EIBT”) disclosed in WO02/103747 (A1), by Zajfman et al. In an EIBT, ions generally oscillate backwards and forwards along a linear path, so such an ion trap is also referred to as a “Linear Electrostatic Ion Trap”.
WO2011/086430, by Verenchikov, discloses an apparatus and operation method for an electrostatic trap mass spectrometer which involves measuring the frequency of multiple isochronous ionic oscillations. For improving throughput and space charge capacity, the trap is substantially extended in one Z-direction forming a reproduced two-dimensional field. Multiple geometries are provided for trap Z-extension. The throughput of the analysis is improved by multiplexing electrostatic traps. This document also suggests that frequency analysis can be done either by Wavelet-fit analysis of the image current signal or by using a time-of-flight detector for sampling a small portion of ions per oscillation.
GB1103361.0, currently unpublished, describes another electrostatic trap mass spectrometer.
US2011/0240845 (also see CN101752179), by Li Ding (one of the present inventors), discloses a mass spectrometric analyser and an analysis method based on the detection of ion image current. The method in one embodiment includes using electrostatic reflectors or electrostatic deflectors to enable pulsed ions to move periodically for multiple times in the analyser, forming time focusing in a portion of the ion flight region thereof, and forming an confined ion beam in space; enabling the ion beam to pass through multiple tubular image current detectors arranged in series along an axial direction of the ion beam periodically, using a low-noise electronic amplification device to detect image currents picked up by the multiple tubular detectors differentially, and using a data conversion method, such as a least square regression, to acquire a mass spectrum.
The inventors have observed that an image charge/current signal obtained using an ion trap mass spectrometer is often not perfectly harmonic. In other words, an image charge/current signal obtained using an ion trap mass spectrometer often has a waveform of sharp pulses in the time domain, which can result in the image charge/current signal having a plurality of harmonic components in the frequency domain.
When an image charge/current signal representative of trapped ions having different mass/charge ratios undergoing oscillatory motion is converted to the frequency domain, e.g. using a Fourier transform, the inventors have observed that, if a plurality of harmonic components are present, each harmonic component is expressed as a set of peaks, with each peak in the set being caused by trapped ions having a different mass/charge ratio (i.e. a different ion species). If the trapped ions have a narrow range of mass/charge ratios, then each harmonic component will be expressed as a set of closely spaced peaks which can easily be identified. However, if the trapped ions have a wide range of mass/charge ratios, then each harmonic component will be expressed as a set of widely spaced peaks which may overlap with each other. Overlapping harmonic peaks can make it difficult to obtain useful information regarding the mass/charge ratio distribution of trapped ions without limiting the range of mass/charge ratios of ions used to obtain the image charge/current signal. These difficulties are described in more detail below, with reference to FIGS. 1a-c. 
Attempts have been made to address these difficulties that can be caused by a plurality of harmonic components being contained in an image charge/current signal obtained using an ion trap mass spectrometer. However, such attempts tend to involve computationally intensive methods.
For example, “Multi-ion quantitative mass spectrometry by orthogonal projection method with periodic signal of electrostatic ion beam trap”, Qi Sun, Changxin Gu and Li Ding (one of the inventors), J. Mass. Spectrum. 2011, 46, 417-424, discloses analysing image charge/current signals using an “orthogonal projection method” to provide a more readable spectrum. However, the method proposed by this paper is computationally intensive.
As another example, “A comb-sampling method for enhanced mass analysis in linear electrostatic ion traps”, J. B. Greenwood et al, Review of Scientific Instruments, 82, 043103 (2011) discloses a “comb-sampling” algorithm for extracting spectral information from signal acquired by pickup-electrodes from the image-charge of ion bunches oscillating in a linear electrostatic trap. Again, the method proposed by this paper is computationally intensive.
The present invention has been devised in light of these considerations.